The vulgar are never really happy with their luck, even when it is best, or unhappy with their intellect, even when it is worst.

[Vulgaridad es no estar contento ninguno con su suerte, aun la mayor, ni descontento de su ingenio, aunque el peor.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 209 (1647) [tr. Maurer (1992)]
    (Source)

Gracian frames this as an old saying. (Source (Spanish)). Alternate translations:

No man is content with his own condition, though it be the best: nor dissatisfied with his wit, though it be the worst.
[Flesher ed. (1685)]

... the common prejudice that any one is satisfied with his fortune, however great, or unsatisfied with his intellect, however poor it is.
[tr. Jacobs (1892)]

None is content with his fortune even though the best, and none is discontented with his mind, even though the worst.
[tr. Fischer (1937)]